Elastic Element Exoskeleton and Method of Using Same

ABSTRACT

Running in a mammal, such as a human, is augmented by adaptively modulating anticipation of maximum leg extension of a mammal when running, and actuating an exoskeletal clutch linked in series to at least one elastic element, wherein the clutch and elastic element form an exoskeleton and are attached in parallel to at least one muscle-tendon unit of a leg of the mammal and span at least one joint of the mammal fitted with the exoskeleton. The exoskeletal clutch is actuated in advance of a predicted maximum extension of the exoskeletal clutch to thereby cause the exoskeletal clutch to lock essentially simultaneously with ground strike by the leg of the mammal. The elastic element is thereby engaged during stance phase of the gait of the mammal while running, and subsequently is disengaged prior to or during the swing phase of the gait of the mammal, thereby augmenting running of the mammal.

RELATED APPLICATION(S)

This application is a divisional of U.S. application Ser. No.13/774,774, filed Feb. 22, 2013, which claims the benefit of U.S.Provisional Application No. 61/602,851, filed on Feb. 24, 2012.

The entire teachings of the above applications are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

Metabolic augmentation of human locomotion has proved an elusive goal.Although a number of exoskeletons have been built, none has demonstrateda significant reduction in metabolic demand of locomotion. Exoskeletonsmay loosely be classified as intended to augment human capabilities,such as load capacity or ambulatory speed, or to increase humanendurance, by lowering the metabolic demand of the given activity. Forexample, an exoskeletal device intended to reduce the metabolic demandof movement may alternatively permit the execution of that movement athigher speed for a given metabolic demand. Other devices, intended torestore lost functionality, may also be thought of as exoskeletons.

Exoskeletons are classified as passive, quasi-passive or active, basedon the usage of power. Passive exoskeletons require no energy source andgenerally consist of linkages, springs, and dampers. They typically relyon mechanisms, are less robust and, consequently, may result in behaviorthat may lead or lag what is intended. Active devices, in contrast, addenergy to the gait cycle, usually through motors or hydraulic cylinders.Active systems are often limited by weight limitations necessary tominimize changes in momentum that occur during gait cycles, particularlyduring running. Quasi-passive devices lie between passive and activedevices, being unable to inject energy into the gait cycle, butnonetheless requiring a power supply, usually to operate electroniccontrol systems, clutches or variable dampers. Typically, although notnecessarily, the power requirements of a quasi-passive device arerelatively low. Further, exoskeletons, whether active, passive orquasi-passive, may be described as primarily acting in series or inparallel with a subject's limbs.

Moreover, the mechanics of walking and running are significantlydifferent. Specifically, walking resembles, and can be modeled as, aninverted pendulum wherein kinetic and gravitational potential energiesare substantially out of phase. During running, however, kinetic andgravitational potential energies are almost perfectly in phase, wherebythe center of mass and, thus, potential energy are highest atessentially the same time as each other. In other words, during running,elastic potential energy is stored in muscle-tendon units in a cyclethat is out of phase with kinetic and gravitational potential energy.Generally, active, passive and quasi-passive exoskeletons do notaccommodate the running gait of a legged animal, such as a mammal,including, for example, a human wherein the center of mass and, thus,potential energy is highest at approximately the same time velocity and,thus, kinetic energy is highest (in phase), and, whereby elasticpotential energy must be stored out of phase with kinetic andgravitational potential energy.

Therefore, a need exists for an exoskeleton that can augment running ina mammal, such as a human, that overcomes or minimizes theabove-referenced difficulties.

SUMMARY OF THE INVENTION

The present invention is directed to a method for augmenting running ina mammal, such as a human, and to a clutched elastic element exoskeletonthat employs the method of the invention.

In one embodiment, the method for augmenting running in a mammalincludes the steps of adaptively modulating anticipation of a maximumextension of an exoskeletal clutch attached to the leg of a mammal whenrunning. The exoskeletal clutch is linked to at least one elasticelement to form an exoskeleton, wherein the clutch and the elasticelement are attached in parallel to at least one muscle-tendon unit ofthe leg of the mammal. The exoskeletal clutch is actuated in advance ofa predicted maximum extension of the exoskeletal clutch, to therebycause the exoskeletal clutch to lock essentially simultaneously with theground strike by the mammal, whereby the elastic element is engagedduring a stance phase of the gate of the mammal while running. Theelastic element is disengaged prior to or during a swing phase of thegait of the mammal, thereby augmenting running of the mammal.

In one particular embodiment of the method of the invention, adaptivelymodulating maximum extension of the exoskeletal clutch includescorrelating a position of the exoskeletal clutch and an angular velocityof the exoskeleton in a sagittal plane of the mammal with a phase of thegait cycle of the mammal while running, to thereby estimate thepredicted maximum extension of the exoskeletal clutch prior to groundstrike of the leg of the mammal while running.

In a further embodiment of the invention, adaptively modulatinganticipation of maximum extension of the exoskeletal clutch furtherincludes, upon or after estimating the predicted maximum extension,correlating past positions of the exoskeletal clutch during a terminalswing phase with each other to thereby predict maximum extension of theexoskeletal clutch while running.

One clutched elastic element exoskeleton of the invention includes alongitudinal harness, including a proximal component and a distalcomponent. A rotary clutch assembly is linked in series to at least oneelastic element, wherein the rotary clutch assembly and the elasticelement span the proximal and distal components, and wherein a majorlongitudinal axis of each elastic element extends through and isrotatable about a center of rotation at the rotary clutch.

The invention has many advantages. For example, by adaptively modulatinganticipation of maximum extension of an exoskeletal clutch attached tothe leg of the mammal when running, the clutch employed by the methodcan be locked essentially simultaneously with ground strike of the leg,thereby maximizing storage of potential energy upon and after groundstrike. Further, as a quasi-passive device, use of heavy motors andexternal energy storage is avoided, thereby minimizing energy loss bychanges in momentum associated with leg movement while running. Further,the method and apparatus of the invention accommodate changes in strideassociated with changes: i) from running to walking and the reverse; ii)in velocity; and iii) to changes in terrain, including stairs and ramps.Further, the advantages of the invention are not limited by anyparticular stride. For example, when the mammal is a human, the methodand device of the invention are not impaired by whether the humansubject runs by striking the ground first with the heel or ball of thefoot.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one embodiment of the exoskeleton of theinvention.

FIG. 2 is a side view of the exoskeleton shown in FIG. 1 while beingworn by a subject at toe-off while walking.

FIG. 3 is another side view of the exoskeleton shown in FIG. 1 at heelstrike by a subject.

FIG. 4 is a perspective view of one embodiment of a left rotary clutch(for a left knee) of the invention.

FIG. 5 is a perspective view of a portion of an interior of the leftrotary clutch showing the rotary clutch plate and translating clutchplate.

FIG. 6 is a perspective view of a portion of the interior of the rotaryclutch of FIG. 4 showing the ring gear, planet gears, sun bearing andtranslating clutch plate.

FIG. 7 is a perspective view of a portion of the interior of rotaryclutch of FIG. 4 showing the ring gear, planet gears, sun gear androtating clutch plate.

FIGS. 8 and 8A are plan and detail views of the interior of the rotaryclutch of FIG. 4 showing the ring gear, planet gears and sun gear.

FIGS. 9 and 9A are perspective and detail views of a translating clutchplate of the rotary clutch of the invention.

FIG. 10A is an exploded view of one embodiment of a right rotary clutch(for a right knee) of the invention.

FIG. 10B is a perspective view of the rotary clutch shown in FIG. 10A.

FIG. 11 is a side view, in part, of a rotary clutch of the inventionwith leaf springs extending therefrom.

FIGS. 12A and 12B are exploded and perspective views, respectfully, ofanother embodiment of a rotary clutch of the invention, lacking a gearbox.

FIG. 13 is a plan view of one embodiment of a circuit board employed bya rotary clutch of the invention.

FIGS. 14A through 14F are schematic representations of various exemplaryembodiments of exoskeletons of the invention employing a rotary clutch.

FIG. 14G is a schematic illustration of an example embodiment of anexoskeleton of the invention employing a rotary clutch and having aposterior configuration.

FIGS. 15A through 15E are schematic representations of various exemplaryembodiments of harnesses employed to mount proximal portions ofexoskeletons of the invention to a subject.

FIGS. 16A through 16E are schematic representations of various exemplaryembodiments of mechanisms for mounting distal portions of exoskeletonsof the invention to a subject.

FIGS. 17A through 17C are schematic representations of one exemplarymechanism for mounting an exoskeleton to a foot of a subject and theposition of that mechanism during a stance phase of a stride of thesubject.

FIGS. 18A through 18C are schematic representations of positions of theexoskeleton of the invention in late swing, heel strike and stancepositions during running.

FIGS. 19A-19C depict program flow through the framework used to controlthe exoskeletal knee joint of the invention. The solenoid is activatedduring the doubly-circled states. Arrows exiting to the right indicateatypical paths, which at least briefly shut down the normal controlloop. An interrupt (not shown) triggered by pressing the kill buttoncauses an immediate soft kill.

FIG. 20 is another depiction of the program flow of FIGS. 19A-19C.

FIG. 21 is a depiction, in conjunction with FIG. 20, of events used toidentify and act on phases of the gait cycle. Biological hip and kneedata is stereotyped. More than one gait cycle is shown to clarify theperiodic nature of this action.

FIG. 22 is a simulation of solenoid latency compensation predicting peakknee extension. Actual exoskeletal knee data recorded at 800 samples persecond is used. The bold region represents the window of data used inthe final iteration before firing the solenoid. The firing time isdenoted by a circle while the predicted extremum is denoted by a square.Data from time after the solenoid firing is dashed. Note thecorrespondence between the predicted and actual times of peak kneeextension.

FIG. 23 is another depiction of the simulation described with respect toFIG. 22.

FIG. 24 is a plot of metabolic demand calculation.

FIG. 25 is a depiction of biological (dark) and exoskeletal (light)generative phase knee stiffness for several subjects in the inactive andactive conditions.

DETAILED DESCRIPTION OF THE INVENTION

The invention generally is directed to a method for augmenting runningin a mammal, such as a human, and to a clutched elastic elementexoskeleton that can employ the method of the invention.

The method for augmenting running in a mammal and the clutched elasticexoskeleton of the invention can apply relatively high torque with highresolution to a joint of the mammal during running while employingrelatively low mass, thereby overcoming the problems associated with therelatively high mass of active devices and the delayed reaction time ofpassive devices. Further, the method and apparatus of the invention donot depend upon any particular configuration of attachment to the mammalsubject, and accommodate changes in stride and transitions betweenwalking and running, inclines and declines of surfaces, and ascent anddescent of stairs.

One particular embodiment of a clutched elastic element exoskeleton isshown in FIGS. 1-3. As shown therein, exoskeleton 10 includeslongitudinal harness 12. Longitudinal harness 12 includes proximalcomponent 14 and distal component 16 hinged to proximal component 14.Longitudinal harness 12 can be formed of, for example, suitableconventional components, such as leather, laminate polymer composites,etc. Hinges 18 link a distal end of proximal component 14 to a proximalend of distal component 16 on either side of longitudinal harness 12.When fitted to a subject, such as a human, as shown in FIGS. 2 and 3,the axes of rotation of hinges 18 (not shown in FIG. 2) of exoskeleton10 will typically be polycentric with motion compatible with that ofknee 22 of subject 20. Alternatively, in an embodiment not shown,proximal component 14 and distal component 16 do not need to be hinged,other than by virtue of elastic elements 24, 26 and clutch 32. Straps19, 21 about the thigh and calf of subject 20 stabilize proximalcomponent 14 and distal component 16, respectfully, during use.

Proximal elastic element 24 and distal elastic element 26 are connectedto proximal component 14 and distal component 16, respectively, oflongitudinal harness 12 at proximal hinge 28 and distal hinge 30. Hinges28, 30 rotate about respective axes that are substantially parallel toan axis of rotation of knee 22.

Suitable elastic elements typically are of a type known to those in theart and include, for example, at least one member selected from thegroup consisting of leaf springs, compression springs and tensionsprings. As shown in FIGS. 1-3, proximal elastic element 24 and distalelastic element 26 are leaf springs. Leaf springs 24, 26 are formed of asuitable material, such as is known in the art, including, for example,fiber glass and carbon fiber.

Proximal leaf spring 24 and distal leaf spring 26 are linked by rotaryclutch 32. Rotary clutch 32 is linked to proximal leaf spring 24 anddistal leaf spring 26 at proximal mount 34 and distal mount 36,respectively. As can be seen in FIGS. 6 and 10A, proximal mount 34 isattached to medial housing 38 a and lateral housing 38 b by pin 35 andscrews 37. Distal mount 36 is fixed to distal ears 40 a, 40 b and ringgear 42 by pin 39 and screws 41. As can be seen in FIG. 11, majorlongitudinal axes 44, 46 of proximal and distal leaf springs 24, 26extend through and are rotatable about central axis 48, or center ofrotation, of rotary clutch 32.

Returning to FIGS. 4-10B, distal ears 40 a, 40 b secure ring gear 42that, in turn, directs rotation of planet gears 50, 78 and sun gear 52which reside within ring gear 42. Planet gears 50, 78 are stabilized byplanet bearings 51. Sun gear 52 is fixed to rotating clutch plate 54 byflathead screws 56 and stabilized by sun bearing 57 b. Ring gear 42,planet gears 50, 78 and sun gear 52 preferably are formed of a suitablematerial, such as titanium.

Ball bearings 57 a, 57 b support rotating clutch plate 54 and sun gear52. Retaining ring 53 retains sun gear 52 in position relative tobearing 57 b. Sun gear 52 is hollow, allowing solenoid 66, whichactuates the clutch, to be placed within sun gear 52. Radial and axialforces are borne by a pair of opposing angular contact ring bearings 43a, 43 b which support distal ears 40 a, 40 b and ring gear 42. Medialhousing 38 a includes linear plain bearings 61 which support translatingclutch plate 58 and are located between the planetary gears 50, 78 (SeeFIG. 6).

Translating clutch plate 58 includes legs 60 that extend through medialhousing 38 a and are fixed to solenoid mount 62 by screws 64. Rotaryclutch plate 54 and translating clutch plate 58 preferably are formed ofa suitable material, such as titanium. Other remaining components ofrotary clutch typically are formed of a suitable material known in theart, such as aluminum. Solenoid 66 is fixed to solenoid mount 62 andextends in non-interfering relation through sun gear 52 to solenoidplunger 68. Solenoid return spring 70 biases solenoid plunger 68 awayfrom solenoid 66. Solenoid plunger 68 is rigidly fixed by screw 73 tomedial cap 72 which, in turn, is fixed to medial housing 38 a by screws74. Optical encoder disk 76 is fixed to long planet gear 78 by E-clip80. Circuit board 82 and lithium ion battery 84 are fitted withinlateral cap 86. Light pipes 88 and right angle light pipe 90 are alsofitted at lateral cap 86. Right angle light pipe 90 is employed toindicate that the device is on, light pipes 88 are employed fordiagnostic purposes. Lateral cap 86 is fixed to lateral housing 38 b byflathead screws 92. Hard stop 93 is secured to lateral housing 38 b byscrews 95, as shown in FIG. 10A, and is employed to prevent bothhyperextension and collision of mounts 34, 36. To this end, hard stop 93may be engaged on either side by a tab 91 (FIG. 10A) protruding from sungear 52.

As can be seen in FIG. 11, upon actuation, solenoid 66 will draw plunger68 within solenoid 66 in a direction opposite that of bias provided bysolenoid-return spring 70. Solenoid mount 62, in turn, is directed bymovement of solenoid 66 along plunger 68 that moves translating clutchplate 58 into mating relation with rotating clutch plate 54. Engagementof translating clutch plate 58 with rotating clutch plate 54 byactuation of solenoid 68 causes rotation of rotating clutch plate 54consequent to rotation of planetary gears 50, 78 to stop, therebypreventing further rotation of distal mount 36 relative to proximalmount 34 about a central axis of rotary clutch 32. Terminating actuationof solenoid 66 causes plunger 68 to move outwardly from within solenoid66 by virtue of bias provided by solenoid return spring 70, therebycausing translating clutch plate 58, which is slidably engaged withmedial housing 38 a, to move away from rotating clutch plate 54, therebydisengaging translating clutch plate 58 from rotating clutch plate 54and restoring freedom of rotation of distal mount 36 about a centralaxis of rotary clutch 32 relative to proximal mount 34.

As can be seen in FIGS. 9 and 9A, and in FIG. 5, teeth 94 of translatingclutch plate 58 and rotating clutch plate 54 preferably have a matingsawtooth, rather than a square tooth, or castle, configuration in orderto facilitate alignment during relative movement of the translating androtating clutch plates 58, 54. Further, the sawtooth configurationtypically is asymmetric, as shown in FIG. 9A, with the leading edgehaving an angle, for example, of exactly or very nearly 90 degrees inorder to maximize holding tongue.

Circuit board 82 is powered by lithium-ion battery 84 (FIG. 10A) andincludes, as can be seen in FIG. 13, battery protection unit 100,reflective optical encoder 102, inertial measurement unit 104, opticalbreak-beam 106, microcontroller 108, light emitting diode (LED) drivers110, USB interface 112, micro SD card 114, real-time clock 116, solenoiddriver 118, and switching power supply 120. Battery protection unit 100modulates battery charging and monitors for potentially hazardousbattery conditions. Inertial measurement unit 104 includes a gyroscopeand an accelerometer that are employed as secondary gait sensors.Solenoid drive 118 is an electronic circuit that provides current tosolenoid 66 (FIG. 10A). Reflective optical encoder 102 is a sensor thatis employed to determine angular position of rotary clutch 32 (FIG.10A). Optical break-beam 106 is a sensor employed to determine positionof solenoid 66 (FIG. 10A). Microcontroller 108 is the main computerprocessor of rotary clutch 32 (FIG. 10A). LED drivers 110 are electroniccircuits that provide current to diagnostic LEDs that transmit light tothe outside of the lateral cap. USB interface 112 is an electroniccircuit employed to transmit detailed diagnostics and logs over USBconnector 112 a, shown in FIG. 5 and FIG. 13, and is also employed toreprogram microcontroller 108, as necessary. Micro SD card 114 isemployed as memory to store logs. Real-time clock 116 is an electroniccircuit employed to keep time, even when rotary clutch 32 (FIG. 10A) isnot in use, to thereby accurately time-stamp logs. Switching powersupply 120 is an electronic circuit which regulates voltage toacceptable levels for various components.

When in use, proximal component of longitudinal harness is strapped tothigh member 122 of the subject 20, such as a human subject, as shown inFIGS. 2 and 3. Distal component 16 is strapped to a lower leg, such asbelow calf 124 of subject 20. Hinges 18 transversely span knee 22 ofsubject 20. The axes of rotation of hinges 18 are generally polycentricconsistent with knee 22.

As shown in FIGS. 14A-14C, in alternate embodiments, proximal component126 can be fitted to hip 128 of subject 20, whereby hinge 28 at proximalcomponent 126 can be co-axial or polyaxial relative to hip 128 ofsubject 20. In still another embodiment, shown in FIGS. 14D-14F,proximal component 126 is fixed to torso 130 or chest of subject 20,whereby hinge 28 at proximal component 126 is above the axis of rotationof hip 128, and proximal leaf spring 24 spans hip 128 of subject 20. Asshown in FIGS. 14A-14C proximal component 126 is fixed at or below hips128 of subject 20, while distal component 134 is fixed at or below ankle132 of subject, whereby clutched elastic element exoskeleton 10 spansone, two or three of ankle 132, knee 22 and hip 128 joints of subject20. In another embodiment (FIGS. 14E-14F), clutched elastic elementexoskeleton 10 spans all three of ankle 132, knee joint 22 and hip joint128. FIG. 14G shows an embodiment that has a posterior configuration andwhich is a variant of the embodiment of FIG. 14F. As shown, the proximalcomponent 126 is fixed to a posterior or back of torso 130 of subject20. FIGS. 15A-15E show various harnesses that can be employed to supporthinge 28 at proximal component 126 to subject 20 at or above hip 128.

In other embodiments, shown in FIGS. 16A-16E, distal component 134 canbe fitted to subject 20 at or below the ankle 132. For example, as shownin FIGS. 16A-16B, where the distal component 134 is fitted at ankle 132,hinge 30 can be can be fixed to the shin so that spring 26 is notaffected by rotation of ankle 132. In one embodiment, shown in FIG. 16C,distal component 135 can include rigid attachment 138 attached at heel136 of subject 20, with an offset to be approximately concentric toankle 132 so that the spring effectively does not span the ankle. Inother embodiments, shown in FIGS. 16D-16E, where distal components 137,141, respectively, are fitted below ankle 132, distal leaf spring 26effectively spans ankle 132. In the embodiments shown in FIGS. 16A-16C,motion of spring 26 is isolated from rotation of ankle 132, either byattaching to shin (FIGS. 16A-16B) or by attaching to foot (FIG. 16C) andplacing a pivot point (e.g., hinge 30) very near the axis of the ankleto minimize the effective moment arm. In the embodiments shown in FIGS.16D-16E, in contrast, movement of ankle 132 causes movement of spring26.

In one embodiment, shown in FIG. 16D, distal component 137 includesrigid attachment 139 at toe 140 of the subject 20. This embodimentforces subject 20, when running, to lead with toe 140 on ground strikeof subject 20. This embodiment is particularly useful for augmentedhopping.

In still another embodiment, shown in FIG. 16E, two degrees of freedomare provided at the point of contact between distal component 141 andthe subject. In this embodiment, direct ground contact is permittedindependent of foot position, thereby allowing normal heel strike andtoe-off while loading directly into the ground during running.Specifically, longitudinal movement is permitted between pin 142, fixedat heel 143, and holster 144 of distal component 141, and rotationalmovement is permitted about pivot point 146 of distal component 141.

One specific embodiment of the schematic representation of FIG. 16E isshown in FIGS. 17A-17C, whereby articulated distal attachment 150 allowsspring 26 to load through the ground even as foot 152 moves around it.Return springs 150 a and 150 b are located at the heel of the subjectand along a linear bearing in the attachment, respectively. Atheel-strike, spring 26 contacts ground below toe 140. As foot 152 rollsforward, linear and rotary joints allow spring contact point 154 toremain stationary until toe-off, after which return springs 150 a and150 b contract, resulting in movement of the device back to the positionshown in FIG. 17A.

During use, elastic element exoskeleton 10, shown in FIGS. 2 and 3, isfixed at proximal component 14 of longitudinal harness 12 to thigh 122of subject 20 and at distal component 16 to lower leg 124 of subject 20above ankle joint 132. Microcontroller 108 (FIG. 13) of rotary clutch 32adaptively modulates anticipation of maximum extension of rotary clutch32 while subject 20 is running. The term “adaptively modulatinganticipation” is also referred to, and has the same meaning as“adaptively anticipating”. In one embodiment, adaptively modulatinganticipation of (i.e., adaptively anticipating) maximum leg extension ofrotary clutch 32 includes measuring the angular position of the rotaryclutch 32 by use of optical encoder 106 (FIG. 13) of rotary clutch 32,and the angular velocity of rotary clutch 32 in a sagittal plane insubject 20 by use of gyroscope of inertial measurement unit 104 (FIG.13). The angular position of rotary clutch 32 and angular velocity ofexoskeleton 10 are correlated with a phase of the gait cycle of subject20 while running, to thereby predict the time of maximum extension ofrotary clutch 32 during late swing phase of the gait cycle of subject20, prior to leg strike of subject 20, while running. Further,adaptively modulating anticipation of maximum leg extension of rotaryclutch 32 can include, upon or after estimating predicted maximum legextension, correlating past positions of the rotary clutch 32 duringterminal swing phase with each other, to thereby predict maximumextension of rotary clutch 32 while subject 20 is running.

FIGS. 18A-18C depict compression of leaf springs 24 and 26 before andduring the stance phase of running, while rotary clutch 32 is locked. Asshown in FIG. 18A, rotary clutch 32 has not yet reached maximumextension in late swing phase. In FIG. 18B, rotary clutch 32 has reachedmaximum extension and has locked essentially simultaneously with kneestrike. As can be seen in FIG. 18C, during stance phase, linear springs24 and 26 store potential energy by flexing. The stored potential energyis released during late stance phase to thereby augment running by thesubject.

Optionally, rotary clutch 32 is disengaged by correlating the positionof exoskeletal and angular velocity of exoskeleton 10 with a mid-stanceor terminal stance phase of the gait cycle while subject 20 is running,and actuating disengagement of the rotary clutch 32 at mid-stance phase,as predicted by the correlation.

As a further option, correlating past positions of rotary clutch 32 topredict maximum extension of the rotary clutch is conducted by applyinga latency compensation algorithm. In one embodiment, the latencycompensation algorithm includes a quadratic least squares analysis. Inan alternate embodiment, the latency compensation algorithm includesfitting differentials of encoder readings to a line and seeking a zerocrossing, as shown in FIG. 23, and described below.

In another embodiment, shown in FIG. 12A, clutch 200 lacks a gear box,such as the ring, planets and sun gear assembly of the embodiment shownin FIGS. 4-11. Rotary clutch 200, as shown, is a left knee clutch.Clutch housing 201 (FIG. 12B) includes lateral clutch housing 202 andmedial clutch housing 204 (FIG. 12A). Lithium polymer battery 206 fitswithin lateral clutch housing 202 and medial clutch housing 204. Lithiumpolymer battery 206 is electrically connected to circuit board 208.Power button 210 extends through lateral clutch housing 202 and islinked to circuit board 208. Inertial measurement unit 212 is mounted toframe 214. Circuit board 208, in turn, is mounted within medial clutchhousing 204. Proximal mount 216 is mounted to a linear spring (notshown), as described above. Distal mount 218 is mounted to distal linearspring (not shown) as also described above. Solenoid 220 extends throughbearings 222, rotating clutch plate 230, distal mount 218 and encoderdisc 226. Encoder disc 226 is fitted to encoder reader 228 which, inturn, is mounted within medial clutch housing 204 and electricallyconnected to circuit board 208. Rotating clutch plate 230 is fixed toproximal mount 216. Encoder disc 226 is fixed to distal mount 218 androtates with distal mount 218. Solenoid 220 is fixed within rotatingclutch plate 230. Translating clutch plate 232 is seated within housing242. Preload nut 234 and belleville washer 236 are seated within housing242. Preload nut 234 secures rotating clutch plate 230 in place whileallowing rotation. Proximal mount 216 is fixed to housing 242. Solenoidplunger 238 is fixed to plunger mount 240 and extends through plungermount 240, preload nut 234, belleville washer 236, and also extendswithin solenoid 220. Actuation of clutch 200 causes plunger 238 to movewithin solenoid 220, thereby directing plunger mount 240 to movetranslating clutch plate 232 into engagement with rotating clutch plate230. As with the rotary clutch described above, translating clutch plate232 and rotating clutch plate 230 are formed of a suitable material,such as titanium. A fully-assembled left knee clutch is shown in FIG.12B. Actuation of clutch 200 is triggered in the same manner asdescribed with respect to the clutch and the remainder of theexoskeleton shown in FIG. 1, described supra.

The following representation is an exemplary embodiment of the method ofthe invention as applied to one embodiment of the exoskeleton of theinvention. The description and results set forth should not beconsidered limiting in any way.

EXEMPLIFICATION Electronics and Instrumentation

To increase reliability and facilitate maintenance, the system isdesigned with a minimum number of routed wires. To this end, allelectronics are packaged together within a cap-shaped subassembly whichattaches to the lateral face of the proximal assembly and is easilyremoved for maintenance. This lateral subassembly contains a 2000 mAhlithium polymer battery cell and the circuit board, both fixed to amilled aluminum housing. The circuit board is annular, to accommodatethe last 2 mm of solenoid travel through the center of the board. Allsensors (Table 4-1) are mounted directly to the circuit board and, wherenecessary, interface optically to appropriate mechanical transducerswithin the clutch. Only a single pair of wires, connecting the solenoidto the circuit board, links the lateral assembly to the body of theclutch. A floorplan of the circuit board is shown in FIG. 13.

An AtMega168PA AVR microcontroller operating at 12 MHz controls theclutch, using a development framework described below. A set of sixteenLEDs, directed to the face of the lateral subassembly by light pipes,provides visual indication of state. More complete diagnostic logs areavailable through USB tethering or may be recorded on an onboard MicroSDcard for later analysis. The microcontroller may be reprogrammed overUSB.

A three degree of freedom inertial unit comprising a dual-axis MEMsaccelerometer and a MEMs gyroscope provides acceleration and rotationrate sensing within the sagittal plane. The accelerometers are primarilyused to assess heel strike. Because the circuit board is fixed to theproximal assembly, the gyroscope is indicative of hip rotationalvelocity and is used to assess midswing and midstance. Rotation rate inmidswing is particularly informative as an indication of runningvelocity.

TABLE 1 Sensors used in the exoskeletal knee. Measurement PartTechnology Interface Resolution Range Bandwidth Anterior-PosteriorAcceleration ADIS16006 MEMS Accelerometer SPI 0.038 m/s²  ±49 m/s^(s)100 Hz Superior-Inferior Acceleration Sagittal Plane Angular RateADIS16100 MEMS Gyroscope SPI 1.12° ±1380°/s 185 Hz Exoskeletal KneeAngle E4P (Disk) Reflective Encoder SPI 0.068° 0-135° AEDR (Reader)LS7336R (Counter) Clutch Engagement Distance EE-SX1109 Break Beam Analog 0.1 mm  0-2 mm

Rotation of the clutch is measured using a reflective optical encoder.The quadrature phase disk is mounted to one of the planets rather thandirectly to the distal subassembly, both to accommodate the solenoid atthe center of the device and to obtain an effective increase inresolution from the higher speed of the planets. It aligns with thePrinted Circuit Board (PCB)-mounted reader when the lateral subassemblyis installed.

Solenoid position feedback is obtained from an infrared break beamsensor soldered to the PCB interacting with an aluminum flag machinedinto the solenoid mount. This flag is dimensioned such that the sensorsaturates when the solenoid mount is completely disengaged, but providesanalog sensing over the final 2 mm of engagement, including any partialtooth engagements. This sensor is non-linear and exhibits slighthysteresis. For practical purposes, it offers 0.1 mm resolution.

Three power rails are generated from the 3-4.2V battery supply byswitching converters. A 3.3V rail, produced by a four switch buck-boostconverter, powers all onboard logic and most sensors. A 5V rail,produced by a boost converter, is needed to power the optical encoderand gyroscope, as 3.3V variants are unavailable. Finally, a 24V rail,produced by a boost converter, is used to power the solenoid. A 3V lowdropout linear regulator is placed between the 3.3V rail and theaccelerometer to eliminate power supply ripple, to which this sensor isparticularly sensitive. The battery is charged over USB and is protectedin hardware from over-current, over-voltage, and under-voltage. Toconserve battery, the system is powered down by software after a periodof inactivity on all sensors.

TABLE 2 Typical power consumption of exoskeletal knee clutch. VBattpower and switching converter efficiencies are calculated assumingnominal 3.7 V battery. (a) Electronics power on battery, 3.3 V, and 5 Vrails V_(Batt) 3.3 V 5 V Battery Management 14 μA Microcontroller 5 mAAccelerometer 1.5 mA Gyroscope 7.1 mA Encoder Reader 15 mA 2.1 mAEncoder Counter 200 μA Optical Break Beam 8 mA Real Time Clock 15 nA 80μA LED Drivers 390 μA Total Current 14 μA 30.2 mA 9.2 mA Total Power 52μW 100 mW 46 mW (b) Typical solenoid power on 24 V rail Closing DutyCycle 1.00 Closing Power 13 W Closing Time 28.6 ms Closing Energy 375 mJHolding Duty Cycle 0.24 Holding Power 725 mW Holding Time (Typ.) 300 msHolding Energy (Typ.) 330 mJ Total Energy (Typ.) 705 mJ Stride Period(Typ.) 1.3 s Total Power (Typ.) 540 mW (c) Total power drawn frombattery Rail P_(load) Efficiency P_(battery) V_(Batt) 52 μW 100%  52 μW3.3 V 100 mW 82% 122 mW 5 V 46 mW 92% 50 mW 24 V 540 mW 79% 685 mW 857mW

Optimal control of the exoskeletal knee joint produces full engagementat the time of maximum knee extension shortly before heel strike andfull disengagement prior to toe off. Ideally, each exoskeletal kneeachieves this independently and requires no extrinsic inputs. Thecontroller is implemented within a framework developed with prostheticand orthotic systems in mind. The control problem itself is divided intotwo primary components: analyzing the gait cycle using kinematic sensingand compensating for the electromechanical latency of the clutch.Additionally, a pulse and hold strategy is implemented to reduce powerconsumption in the solenoid once the clutch is fully engaged.

The control framework, written for the AVR AtMega*8 line ofmicrocontrollers, provides synchronous read-out of all sensors andupdate of all output devices as well as diagnostic and remote controlcapabilities. In particular, it is designed so that the space accessibleto an end user is both easy to develop in and relatively well sandboxed.As this framework provides all low-level functionality, discussion herefocuses on the two primary components of the exoskeleton controlproblem: analyzing the gait cycle using kinematic sensing andcompensating for the electromechanical latency of the clutch.

The framework, shown in FIGS. 19A-19C as configured for the exoskeletalknee clutch, implements synchronous time division for five phases ofoperation occurring in a loop:

-   -   Latch In: All input devices are read into memory.    -   User Space: A state machine is updated based on newly updated        input data. Lead In and Lead Out subphases are executed        immediately before and immediately after the state machine's        update and are suited for filtering inputs and updating        closed-loop controllers independently of the current state.    -   Latch Out: Changes made to output devices during the User Space        phase are applied to hardware.    -   Debugger: A programmable set of data is logged, usually to USB        or onboard memory.    -   Remote Control: A programmable set of memory locations may be        updated, usually over USB. The remote control also provides for        remote soft kill and wake as well as access to a bootloader so        that new system code may be loaded.

Time division is enforced by a timer interrupt and a timeout results inan immediate hard kill, in which all potentially hazardous outputs areturned off and the system is shut down pending a reset from physicalinput or via the remote control. Program flow is blocked until thecompletion of a phase's time division if it completes early,guaranteeing synchronization at the start of each phase.

A soft kill, in which program flow continues, but potentially hazardousoutputs are turned off and the state machine is forced into sleep, maybe entered by pressing a kill switch, by software request during theLatch In, User Space, or Latch Out phases, or by request over the remotecontrol.

Gait Analysis

The framework provides a user-friendly environment for implementing agait analysis state machine. FIGS. 20 and 21 depict a stereotypedrunning gait, including knee and hip angles.

TABLE 3 Variables related to exoskeletal knee control, grouped intodirect sensor readings and calculated internal state. Symbol DescriptionUnits {umlaut over (x)} Anterior-Posterior Acceleration m/s² ÿSuperior-Inferior Acceleration m/s² {dot over (ψ)} Sagittal PlaneAngular Velocity °/s θ Exoskeletal Knee Angle ° λ Scaled Optical BreakBeam Reading Δt_(state) Time Since Last State Change ms Δt_(eta)Predicted Time to Peak Knee Extension ms η Fractional Clutch Engagement

TABLE 4 Constants related to exoskeletal knee control, grouped intointrinsic hardware properties, tunable parameters, and tunable statemachine time constraints Symbol Description Value f Update Frequency 750Hz Δt_(eng) Clutch Engagement Time 30 ms λ_(open) Optical Break BeamOpen Threshold 0.20 λ_(closed) Optical Break Beam Closed Threshold 0.90W Latency Compensation Window Size 32    {dot over (ψ)}_(swing) SagittalPlane Angular Velocity 190°/s Swing Threshold {dot over (ψ)}_(stance)Sagittal Plane Angular Velocity −28°/s Stance Threshold ÿ_(strike)Superior Acceleration Heel Strike 17.2 m/s² Threshold Δθ_(flexion)Hysteresis Width to Detect Peak  2° Knee Flexion Δθ_(swing) Minimum KneeExcursion in Swing 44° D_(open) Solenoid Duty Cycle While Closing 1.00D_(closed) Solenoid Duty Cycle Once Closed 0.24 Δt_(Swing1, max) MaximumTime in Swing 1 200 ms Δt_(Swing2, max) Maximum Time in Swing 2 320 msΔt_(TerminalSwing, max) Maximum Time in Terminal Swing 120 msΔt_(EarlyStance, min) Minimum Time in Early Stance 40 msΔt_(EarlyStance, max) Maximum Time in Early Stance 200 msΔt_(TerminalStance, min) Minimum Time in Terminal Stance 20 ms

Relying exclusively on the onboard sensor measurements introduced inTable 1, a simple state machine (shown in FIG. 20) suffices to interpretthe phases of this gait:

-   -   1. Preswing: Toe-off completes, the knee flexes in order to        minimize its moment of inertia for forward swing, and the hip        begins to flex accelerating the leg forward. Positive rotation        of the gyroscope exceeding ψ_(swing) causes transition to Swing        1.    -   2. Swing 1: The knee continues to flex, eventually achieving        maximum flexion. Once the exoskeletal knee angle has extended        beyond its observed maximum flexion by a hysteresis band        Δθ_(flexion), the state machine advances to Swing 2.    -   3. Swing 2: The knee begins to extend in preparation for heel        strike. The solenoid latency compensation algorithm is        activated. Once the projected time to maximum knee extension Δ        t_(eta) is less than the known clutch engagement time Δ t_(eng)        and the exoskeletal knee angle has decreased by at least Δ        θ_(swing), the solenoid is activated and the state machine        progresses to Terminal Swing.    -   4. Terminal Swing: The clutch engages shortly before the foot        touches the ground. Vertical acceleration in excess of        ÿ_(strike) at impact causes transition to Early Stance.    -   5. Early Stance: The biological knee flexes while the ankle        dorsiflexes, resulting in a shortening effective leg length.        With the clutch engaged, the exoskeletal knee does not flex and        the bow spring bears load, storing energy. The hip flexes,        propelling the body forward. The resulting negative rotation of        the gyroscope in excess of {dot over (ψ)}_(stance) causes        deactivation of the solenoid and transition to Terminal Stance.    -   6. Terminal Stance: The center of mass reaches its lowest point,        after which the biological knee and ankle reverse direction,        resulting in a lengthening effective leg length. Although the        solenoid is off, the clutch is bound by the large applied        torque. As toe off nears, the effective leg length approaches        and eventually exceeds that when the clutch was engaged,        allowing it to relax to its disengaged state. The detection of        this disengagement by the Break beam sensor causes transition to        Preswing.

The solenoid is activated, using a pulse and hold strategy to reducepower consumption, while in the Terminal Swing and Early Stance states.Were the clutch able to engage infinitely quickly, the Swing 2 statecould simply monitor for a minimum in the knee encoder and engage theclutch immediately as it transitions to Terminal Swing. In practice, itis necessary to activate the solenoid slightly prior to the true encoderminimum. This prediction is carried out by the latency compensationalgorithm.

Latency Compensation

A significant latency is associated with the electromechanical systemcomprising the solenoid, return spring, and translating clutch plate.Experimentally, the delay from application of 24V to the solenoid tofull engagement of the clutch is approximately 30 ms. As this time iscomparable to the duration of late swing, it is necessary to compensatefor the electromechanical latency, firing the solenoid early to ensurethat the clutch is fully engaged at the time of maximum knee extension.The latency compensation algorithm in use during the Swing 2 phaseaccomplishes this.

One can consider only late swing phase between peak knee flexion andheel strike (isolated by the technique presented above). During thisphase, knee angle is approximately parabolic so one may fit the observedencoder counts to a second order polynomial with peak knee extension atthe vertex. Using such a continuously generated fit, one can elect tofire the solenoid once the predicted vertex position is less than 30 msin the future.

Unfortunately, the entirety of late swing is not parabolic; aninflection point exists which varies substantially between wearers andis in general difficult to predict or identify. As the region beforethis inflection point would skew the regression, it is advantageous tochoose to fit to a running window rather than to all data in late swing.

While a closed form to a quadratic least squares regression exists (andin fact can be computable only from running sums), there is a simpler,even less computationally expensive solution. Rather than fittingencoder readings to a quadratic and seeking the vertex, one can fitdifferentials of encoder readings to a line and seek the zero crossing.

Let θ_(i) represent the exoskeletal knee angle i samples prior, so thatθ₀ is the current angle and let δθ_(i)=θ_(i)−θ_(i+1) represent adifferential angle between adjacent samples. A sliding window of themost recent W samples may be fit to a line of the form

$\begin{matrix}{{\hat{\delta \;}\theta_{i}} = \frac{{ai} + b}{d}} & (4.1)\end{matrix}$

with coefficients given by

$\begin{matrix}{a = {{{- S_{0}}T_{1}} - {S_{1}T_{0}}}} & (4.2) \\{b = {{S_{2}T_{0}} + {S_{1}T_{1}}}} & (4.3) \\{{d = {{S_{0}S_{2}} - S_{1}^{2}}}{where}} & (4.4) \\{S_{k} = {\sum\limits_{i = 0}^{W - 1}i^{k}}} & (4.5) \\{T_{k} = {\sum\limits_{i = 0}^{W - 1}{i^{k}\delta_{i}}}} & \begin{matrix}(4.6) \\(4.7)\end{matrix}\end{matrix}$

so that

$\begin{matrix}{S_{0} = {{\sum\limits_{i = 0}^{W - 1}1} = W}} & (4.8) \\{S_{1} = {{\sum\limits_{i = 0}^{W - 1}i} = \frac{S_{0}\left( {S_{0} - 1} \right)}{2}}} & (4.9) \\{S_{2} = {{\sum\limits_{i = 0}^{W - 1}i^{2}} = \frac{S_{1}\left( {{2S_{0}} - 1} \right)}{3}}} & (4.10) \\{T_{0} = {{\sum\limits_{i = 0}^{W - 1}{\delta \; \theta_{i}}} = {\theta_{0} - \theta_{W}}}} & (4.11) \\{T_{1} = {{\sum\limits_{i = 0}^{W - 1}{i\; {\delta\theta}_{i}}} = {\left( {\sum\limits_{i = 1}^{W - 1}\theta_{i}} \right) - {\left( {W - 1} \right)\theta_{W}}}}} & (4.12)\end{matrix}$

This fit crosses zero, corresponding to the desired knee extremum, in anumber of samples given by

$\begin{matrix}{{\Delta \; n_{eta}} = {- \left( \frac{b}{a} \right)}} & (4.13)\end{matrix}$

or equivalently

$\begin{matrix}{{\Delta \; t_{eta}} = {{- \left( \frac{1}{f} \right)}\left( \frac{b}{a} \right)}} & (4.14)\end{matrix}$

where f is the sampling frequency.

So, S₁, and S₂ are computable offline and d need not be computed at all.T₀ is trivially calculable and T₁ reduces to a running sum and requiresincremental corrections only for end points of the sliding window. Thus,this approach is extremely inexpensive computationally. FIGS. 22 and 23demonstrate its efficacy with a window 16 samples wide and a look aheadthreshold of 30 samples.

Pulse-and-Hold Solenoid Activation

In order to minimize clutch engagement time, the solenoid may be drivenat D_(open) 100% duty cycle, but it is desirable to reduce this voltageonce the clutch is fully engaged in order to reduce power consumptionand maximize battery life. To this end, a pulse-and-hold strategy isused, settling to an experimentally determined D_(closed)=24% duty cyclesufficient to overcome the return spring once the break beam sensorreports full engagement.

Clinical Testing and Results

In order to determine the effect of parallel elasticity at the kneejoint on running, an experiment was undertaken in which subjects ran ona treadmill with and without the exoskeleton while instrumented forjoint kinematics and kinetics, electromyography, and metabolic demand.

Experimental Design

The proposed exoskeleton provides an elastic element in parallel withthe knee during stance phase, but unfortunately a practical device, likethat outlined above with reference to FIGS. 1-13, also influences thebody in several other ways due to its mass and means of attachment.Additional mass of the exoskeleton has a gravitational effect as hipextensors and knee flexors must lift the mass during early swing and aninertial effect as hip flexors must accelerate the mass during swing.Finally, attachment to the body, as discussed above with reference toFIGS. 14A-17C, is difficult to accomplish without some constriction,which limits range of motion and causes discomfort. In order to isolatethe effect of elasticity, experiments were conducted in threeconditions—control, in which subjects ran in self selected footwear withno experimental apparatus other than those required for instrumentation,inactive, in which subjects wore the investigational knee brace with thepower off, contributing zero stiffness but offering the same secondaryaffects associated with mass and restricted movement, and active, inwhich subjects wore the investigational knee brace with the power on,contributing a non-zero parallel stiffness during stance phase.

TABLE 5 Descriptive measurements of the six recruited subjects and thenumber of steps analyzed for each in the three trials. Fewer steps thanexpected were available for S1 due to lost markers, for S5 due to anequipment failure, and for S6 due to early exhaustion. Leg Length MassCadence Control Inactive Active Age yr Height cm cm kg Steps/s StepsSteps Steps S1 27 175 96 57 172 30 30 11 S2 19 196 107 61 152 50 50 50S3 44 180 99 74 175 50 50 50 S4 25 185 102 82 162 50 50 50 S5 20 180 8577 162 50 50 28 S6 34 170 93 66 166 50 33 37

Six male subjects (Mass 69±8 kg, Height 181±8 cm), described in Table 5,were recruited from a pool of healthy recreational runners having leglength (>90 cm) and circumference (45-55 cm at the thigh, 20-30 cm atthe shin) consistent with the investigational knee brace.

Each subject ran with the device active for a training session of atleast thirty minutes on a day prior to instrumented trials. Subjectstrained initially on open ground then continued on a treadmill wearing afall prevention harness (Bioness, Valencia, Calif., USA). During thistraining session, subjects with a gait insufficiently wide to preventcollision between the braces or with stance knee extension insufficientto ensure disengagement of the clutch were disqualified on the basis ofsafety. During the experimental session, a nominal 0.9 Nm/° elasticelement was used. This relatively small stiffness proved necessary dueto the effects of series compliance in the harness and the tendency ofthe biological knee to resist a stiffer exoskeleton by shiftinganteriorly in the brace.

At the start of the experimental session, each subject's self-selectedstep frequency was measured while running on the treadmill at 3.5 m/swithout the investigational knee brace. The time necessary to complete30 strides was measured by stopwatch after approximately one minute ofrunning. This cadence (166±9 steps/s) was enforced by metronome for allsubsequent trials.

After being instrumented for electromyography and motion capture,subjects then ran on the instrumented treadmill at 3.5 m/s in thecontrol, inactive, and active conditions. Trial order was randomized,excepting that inactive and active conditions were required to beadjacent, so as to require only a single fitting of the investigationaldevice in each session. Each running trial was seven minutes in length,with an intervening rest period of at least as long. Resting metabolismwas also measured for five minutes at both the start and end of theexperimental session. Sessions lasted approximately three hours,including 21 minutes of treadmill running

Instrumentation and Processing

During the experimental session, each subject was instrumented for jointkinematics and kinetics, electromyography, and metabolic demand.

Subject motion was recorded using an 8 camera passive marker motioncapture system (VICON, Oxford, UK). Adhesive-backed reflective markerswere affixed to subjects using a modified Cleveland Clinic marker setfor the pelvis and right leg (Left and right ASIS and Trochanter, threemarker pelvis cluster, four marker thigh cluster, medial and lateralepicondyle, four marker shin cluster, medial and lateral malleolus,calcaneus, foot, fifth metatarsal). For inactive and active trials, thetermination points of the exoskeletal spring were also marked. Motiondata was recorded at 120 Hz and low filtered using a 2^(nd) orderButterworth filter with a 10 Hz cutoff. Ground reaction forces wererecorded at 960 Hz using a dual belt instrumented treadmill (BERTEC,Columbus, Ohio, USA) and low pass filtered using a 2nd order Butterworthfilter with a 35 Hz cutoff. Following calibration using a staticstanding trial, Visual3D (C-Motion Inc, Germantown, Md., USA) modelingsoftware was used to reconstruct joint kinematics and kinetics andcenter of mass trajectories, with right-left leg symmetry assumed.

Fifty steps from each trial were analyzed to determine average leg andjoint stiffness. Due to technical difficulties associated with loss ormigration of motion capture markers and the appearance of false markersdue to reflectivity of the exoskeleton, some motion capture recordingsproved unusable. Consequently, the exact timing of the steps used variesbetween subjects and it was not possible to analyze fifty steps for alltrials, as indicated in Table 5. In general, the earliest availablereconstructions a minimum of one minute into the trial were used, tominimize effects of fatigue.

k_(leg) and k_(vert) were calculated for each step using Equation 1.2and Equation 1.1 with center of mass displacements determined byVisual3D through integration of reaction forces as in G. A. Cavagna,Force Plates as Ergometers, Journal of Applied Philosophy,39(1):174-179, 1975. This effective spring is characterized by k_(vert),given by

$\begin{matrix}{k_{vert} = \frac{F_{z,{peakk}}}{\Delta \; y}} & (1.1)\end{matrix}$

where F_(z,peak) is the maximum vertical component of the groundreaction force and Δy is the vertical displacement of the center ofmass.

Due to the angle subtended, however, the effective leg spring,characterized by k_(leg), is compressed from its rest length L_(o) by ΔLmuch larger than Δy,

so that

$\begin{matrix}{k_{leg} = \frac{F_{z,{peak}}}{\Delta \; L}} & (1.2)\end{matrix}$

Unlike the effective leg spring, the knee and ankle experience differentstiffnesses in absorptive (early) stance and generative (late) stance.Consequently, stiffnesses of these joints were estimated individuallyfor the two phases using

$\begin{matrix}{k_{{joint},{abs}} = \frac{M_{{joint},{peak}} - M_{{joint},{HS}}}{\theta_{{point},{peak}} - \theta_{{joint},{HS}}}} & (5.1) \\{k_{{joint},{gen}} = \frac{M_{{joint},{peak}} - M_{{joint},{TO}}}{\theta_{{point},{peak}} - \theta_{{joint},{TO}}}} & (5.2)\end{matrix}$

where peak represents the instant of peak torque in the joint and HS andTO represent heel-strike and toe-off respectively.

Muscle activation was gauged noninvasively using surfaceelectromyography (EMG), which responds to the membrane potential of amuscle beneath skin. Electrodes were placed above the right soleus,lateral gastrocnemius, tibialis anterior, vastus lateralis, rectusfemoris, biceps femoris, gluteus maximus, and illiopsoas. Wires weretaped to skin and routed an amplifier (Biometrics Ltd, Ladysmith, Va.,USA) clipped to the chest harness containing the cardiopulmonary testsystem. An EMG system with low profile electrodes was used to facilitateplacement around the harness. Nonetheless, placement of the electrode onthe lateral gastrocnemius was suboptimal due to the positioning ofharness straps. A reference electrode was at tached to the wrist. Priorto the first running trial, recordings were made of maximal voluntarycontractions (MVCs) in each muscle.

Electromyography was recorded at 960 Hz then filtered into a lowbandwidth signal indicative of activation by the following filter chain(Robert Merletti, “Standards for Reporting EMG Data,” Technical Report,Politecnico di Tornino, 1999) (Clancy et al, “Sampling, Noise-Reduction,and Amplitude Estimation Issues in Surface Electromyography,” Journal ofElectromyography and Kinesiology, 12:1-16, 2002.) DC block, 60 Hz notchfilter to eliminate mains hum, a 50 ms moving average filter toeliminate motion artifacts, and rectification with a 200 ms movingaverage filter to recover the envelope. Finally, activation for eachmuscle was normalized to the maximum activation seen in stride averagedcontrol trials for that subject.

Metabolic demand was measured noninvasively using a mobilecardiopulmonary exercise test system (VIASYS Healthcare, Yorba Linda,Calif., USA), which measures rates of oxygen consumption and carbondioxide production through a face mask. Once sub-maximal steady statemetabolism was achieved, total metabolic power was deduced from linearexpressions of the form

P=K _(O) ₂ V′ _(O) ₂ +K _(CO) ₂ V′ _(CO) ₂   (5.3)

where V′_(O2) and V′_(CO2) represent average rates of oxygen inhalationand carbon dioxide exhalation and K₀₂ and K_(co2) are constants whichhave been well documented. Brockway's (J. M. Brockway, “Derivation ofFormulae Used to Calculate Energy Expenditure in Man,” Human NutritionClinical Nutrition, 41: 463-471, 1987.) For values K_(O)=16.58 kW/L andK_(CO2)=4.5 kW/L were used. Average rates were calculated over a twominute window during steady state metabolism from 4:00 to 6:00 withineach seven minute trial, as shown in FIG. 24. In addition to the runningconditions, resting metabolic power was also measured with the subjectstanding for five minutes.

Such measures of metabolic power are only valid if the contributions ofanaerobic metabolism are small. This was assured by monitoring the ratioof volume of carbon dioxide exhaled to oxygen inhaled, known as therespiratory exchange ratio. Oxidative metabolism was presumed todominate while this ratio was below 1.1.

More details of the instrumentation used here can be found in (Farris etal., “The Mechanics and Energetics of Human Walking and Running, a JointLevel Perspective,” Journal of the Royal Society interface,9(66):110-118, 2011), in which identical instrumentation and signalprocessing were used, with the omission of electromyography.

Results

Joint and leg stiffnesses calculated for each of the six subjects asdescribed above are presented in Table 6 and Table 7, with averagedstiffnesses presented in Table 9. Subjects S1, S2, S3, and S4 exhibitedsimilar gross kinematics in all three conditions. S5 exhibited similarkinematics in the inactive condition, but transitioned to a toe-strikinggait, with significant ankle plantar flexion at strike in the activecondition. Consequently, S5's active mechanics are not considered inpopulation averages. S6's mechanics are similarly omitted, as he wasvisibly fatigued and failed to complete either the active or inactivetrials.

Metabolic demand, calculated using Equation 5.3 is presented in Table 8for resting, control, inactive, and active conditions, with averageddemand presented in Table 9.

TABLE 6 Effective joint stiffnesses, normalized by subject mass,calculated for the six subjects. Uncertainties reflect the standarddeviation associated with step-to-step variation. Because the controland inactive condition impose zero exoskeletal stiffness and thereforeresult in equal total and biological knee stiffnesses, exoskeletal andbiological contributions at the knee are listed only for the activecondition. Arrows indicate the direction of statistically significantdifferences at the 1% level within a given subject from the control toinactive condition or from the inactive to active condition.Significance was computed using a two sided t-test. Because theuncertainties reported here do not reflect trial-to-trial variation anddue to the large number of comparisons (60) made within this table,these marks should be taken as suggestive of greater trends and nottreated as meaningful in isolation. Control Inactive Activeκ_(ankle,Abs) κ_(ankle,Gen) κ_(ankle,Abs) κ_(ankle,Gen) κ_(ankle,Abs)κ_(ankle,Gen) $\frac{{Nm}\text{/}{kg}}{o}$$\frac{{Nm}\text{/}{kg}}{o}$ $\frac{{Nm}\text{/}{kg}}{o}$$\frac{{Nm}\text{/}{kg}}{o}$ $\frac{{Nm}\text{/}{kg}}{o}$$\frac{{Nm}\text{/}{kg}}{o}$ S1 0.136 ± 0.012 0.078 ± 0.003 0.134 ±0.012  0.087 ± 0.003↑ 0.183 ± 0.022↑ 0.094 ± 0.012  S2 0.182 ± 0.0120.079 ± 0.006 0.165 ± 0.012↓ 0.083 ± 0.005↑ 0.174 ± 0.016↑ 0.085 ±0.007  S3 0.181 ± 0.013 0.077 ± 0.003 0.165 ± 0.010↓ 0.079 ± 0.003↑0.174 ± 0.013↑ 0.086 ± 0.005↑ S4 0.176 ± 0.014 0.065 ± 0.003 0.148 ±0.012↓ 0.069 ± 0.002↑ 0.160 ± 0.011↑ 0.068 ± 0.003  S5 0.153 ± 0.0090.075 ± 0.004 0.138 ± 0.006↓ 0.081 ± 0.005↑ 0.142 ± 0.017  0.096 ±0.004↑ S6 0.110 ± 0.017 0.059 ± 0.007 0.185 ± 0.015↑ 0.085 ± 0.002↑0.123 ± 0.011↓ 0.078 ± 0.004↓ (a) Ankle stiffnesses, measured inabsorptive and generative stance, normalized by body mass ControlInactive κ_(knee,Abs) κ_(knee,Gen) κ_(knee,Abs) κ_(knee,Gen)$\frac{{Nm}\text{/}{kg}}{o}$ $\frac{{Nm}\text{/}{kg}}{o}$$\frac{{Nm}\text{/}{kg}}{o}$ $\frac{{Nm}\text{/}{kg}}{o}$ S1 0.120 ±0.022 0.081 ± 0.007 0.105 ± 0.016↓ 0.066 ± 0.009↓ S2 0.113 ± 0.022 0.106± 0.010 0.111 ± 0.019  0.087 ± 0.011↓ S3 0.132 ± 0.021 0.140 ± 0.0180.106 ± 0.017↓ 0.092 ± 0.005↓ S4 0.088 ± 0.011 0.091 ± 0.007 0.114 ±0.016↑ 0.089 ± 0.007  S5 0.093 ± 0.016 0.077 ± 0.009 0.088 ± 0.012 0.061 ± 0.006↓ S6 0.099 ± 0.021 0.066 ± 0.007 0.145 ± 0.018↑ 0.103 ±0.010↑ Active κ_(knee,Abs) κ_(knee,Gen) κ_(exo,Abs) κ_(exo,Gen)κ_(bioknee,Abs) κ_(bioknee,Gen) $\frac{{Nm}\text{/}{kg}}{o}$$\frac{{Nm}\text{/}{kg}}{o}$ $\frac{{Nm}\text{/}{kg}}{o}$$\frac{{Nm}\text{/}{kg}}{o}$ $\frac{{Nm}\text{/}{kg}}{o}$$\frac{{Nm}\text{/}{kg}}{o}$ S1 0.124 ± 0.023  0.085 ± 0.013↑ 0.019 ±0.005 0.014 ± 0.005 0.108 ± 0.021  0.072 ± 0.013  S2 0.113 ± 0.016 0.089 ± 0.011  0.025 ± 0.006 0.020 ± 0.003 0.089 ± 0.015↓ 0.069 ± 0.011↓S3 0.130 ± 0.024↑ 0.107 ± 0.010↑ 0.017 ± 0.002 0.016 ± 0.002 0.114 ±0.024  0.091 ± 0.009  S4 0.110 ± 0.015  0.088 ± 0.006  0.011 ± 0.0010.010 ± 0.002 0.100 ± 0.015↓ 0.078 ± 0.006↓ S5 0.062 ± 0.014↓ 0.056 ±0.007↓ 0.015 ± 0.062 0.041 ± 0.053 0.060 ± 0.070  0.041 ± 0.019↓ S60.093 ± 0.030↓ 0.073 ± 0.008↓ 0.020 ± 0.027 0.020 ± 0.014 0.070 ± 0.030↓0.056 ± 0.009↓ (b) Knee stiffnesses, measured in absorptive andgenerative stance, normalized by body mass

TABLE 7 Effective leg stiffnesses, normalized by subject mass and leglength, calculated for the six subjects. Uncertainties reflect thestandard deviation associated with step-to- step variation. Arrowsindicate the direction of statistically significant differences at the1% level within a given subject from the control to inactive conditionor from the inactive to active condition. Significance was computedusing a two sided t-test. Because the uncertainties reported here do notreflect trial-to-trial variation and due to the large number ofcomparisons (60) made within this table, these marks should be taken assuggestive of greater trends and not treated as meaningful in isolation.Control Inactive Active k_(leg) k_(vert) k_(leg) k_(vert) k_(leg)k_(vert) $\frac{N\text{/}{kg}}{m\text{/}m}$$\frac{N\text{/}{kg}}{m\text{/}m}$$\frac{N\text{/}{kg}}{m\text{/}m}$$\frac{N\text{/}{kg}}{m\text{/}m}$$\frac{N\text{/}{kg}}{m\text{/}m}$$\frac{N\text{/}{kg}}{m\text{/}m}$ S1 196 ± 14  692 ± 122 204 ± 16 820 ± 165↑ 218 ± 26  793 ± 182 S2 191 ± 16  608 ± 104 198 ± 15  688 ±138↑ 204 ± 9  708 ± 115 S3 241 ± 15  733 ± 100 249 ± 16↑ 815 ± 135↑ 280± 23↑  956 ± 210↑ S4 153 ± 8  499 ± 57  165 ± 11↑ 592 ± 125↑ 155 ± 7↓ 530 ± 52↓ S5 166 ± 16  626 ± 130 169 ± 15  716 ± 260  205 ± 16↑ 694 ±200 S6 182 ± 11  727 ± 115 185 ± 10  664 ± 105  193 ± 15   734 ± 109↑

TABLE 8 Metabolic demands, normalized by subject mass, calculated forthe six subjects. Uncertainties reflect the standard error associatedwith breath-to-breath variation. Arrows indicate the direction ofstatistically significant differences at the 1% level within a givensubject from the control to inactive condition or from the inactive toactive condition. Significance was computed using a two sided z- test.Because the uncertainties reported here do not reflect trial- to-trialvariation, these marks should be taken as suggestive of greater trendsand not treated as meaningful in isolation. Resting Control InactiveActive W/kg W/kg W/kg W/kg S1 1.7 ± 0.2 15.7 ± 0.1 21.2 ± 0.1↑ 20.7 ±0.1↓ S2 1.3 ± 0.2 17.2 ± 0.2 19.0 ± 0.3↑ 19.7 ± 0.3 S3 1.1 ± 0.1 16.6 ±0.1 20.6 ± 0.0↑ 20.3 ± 0.1↓ S4 1.4 ± 0.1 16.6 ± 0.1 19.6 ± 0.1↑ 20.4 ±0.1↑ S5 1.2 ± 0.1 17.2 ± 0.3 16.9 ± 0.1 — S6 1.7 ± 0.1 16.2 ± 0.2 — —

TABLE 9 Mean joint stiffnesses, leg stiffnesses, and metabolic demands.Uncertainties reflect the standard deviation associated withsubject-to-subject variation. Arrows indicate the direction ofstatistically significant differences at the 5% level from the controlto inactive condition or from the inactive to active condition.Significance was determined using a post-hoc paired two-{hacek over(S)}idák-corrected test, following a repeated measures ANOVA. Due toatypical kinematics, results for S5 and S6 are omitted, though S5's dataare used to compare control and inactive conditions; see text fordetails. Control Inactive Active κ_(ankle,Abs)$\left( \frac{{Nm}\text{/}{kg}}{o} \right)$ 0.169 ± 0.022 0.153 ±0.015 0.173 ± 0.010 κ_(ankle,Gen)$\left( \frac{{Nm}\text{/}{kg}}{o} \right)$ 0.075 ± 0.006  0.079 ±0.008↑ 0.083 ± 0.011 κ_(knee,Abs)$\left( \frac{{Nm}\text{/}{kg}}{o} \right)$ 0.113 ± 0.019 0.109 ±0.004 0.119 ± 0.009 κ_(knee,Gen)$\left( \frac{{Nm}\text{/}{kg}}{o} \right)$ 0.105 ± 0.026 0.084 ±0.012 0.092 ± 0.010 κ_(exo,Abs)$\left( \frac{{Nm}\text{/}{kg}}{o} \right)$ — — 0.018 ± 0.006κ_(exo,Gen) $\left( \frac{{Nm}\text{/}{kg}}{o} \right)$ — — 0.015 ±0.004 κ_(bioknee,Abs) $\left( \frac{{Nm}\text{/}{kg}}{o} \right)$0.113 ± 0.019 0.109 ± 0.004 0.102 ± 0.011 κ_(bioknee,Gen)$\left( \frac{{Nm}\text{/}{kg}}{o} \right)$ 0.105 ± 0.026 0.084 ±0.012 0.078 ± 0.010 k_(leg)$\left( \frac{N\text{/}{kg}}{m\text{/}m} \right)$ 195 ± 36  204 ±35  214 ± 52  k_(vert)$\left( \frac{N\text{/}{kg}}{m\text{/}m} \right)$ 633 ± 103  729 ±110↑ 747 ± 178 P_(met) $\left( \frac{W}{kg} \right)$ 16.5 ± 0.7 20.3 ±0.8  20.3 ± 0.4 

Average Mechanics and Metabolic Demand

For each stiffness as well as metabolic demand, a repeated measuresANOVA was conducted to determine significance of trends apparent above.Due to the outlying nature of S6's inactive trial and S5's active trial,their data for all conditions was omitted from this test. For eachstiffness found to vary among the three groups, a post-hoc two-sidedpaired t-test was conducted using {hacek over (S)}idák correction tocompare the control and inactive conditions and inactive and activeconditions, so that P=0.0253 is considered significant.

ANOVA suggests total leg stiffness varies among the conditions (P=0.08),with post-hoc paired t-testing revealing that the observed increase in kleg due to inactive mass is significant (P<0.01), but that nosignificant difference exists between the inactive and activeconditions. This suggestion that increased mass at the knee increasesleg stiffness is interesting, particularly in light of He et al.,“Mechanics of Running Under Simulated Low Gravity,” Journal of AppliedPhysiology, 71:863-870, 1991 finding that leg stiffness does not varywhen gravity is reduced. Moreover, if leg stiffness is normalized bytotal mass rather than by subject mass (as was not necessary in He etal., “Mechanics of Running Under Simulated Low Gravity, Journal ofApplied Physiology, 71:863-870, 1991), no evidence of increase is found.

ANOVA suggests variation in total generative phase knee stiffness(P=0.10) and finds significant variation in biological generative phaseknee stiffness (P=0.04). Post-hoc testing suggests that generative phaseknee stiffness decreases due to the additional mass (P=0.10), but doesnot find evidence of difference between the inactive and activeconditions.

Additionally, a significant variation in ankle stiffness in generation(P=0.02), with post-hoc testing suggesting a difference between thecontrol and inactive conditions (P=0.06) but not between inactive andactive conditions.

A suggestive difference exists in metabolic demand between the controland inactive conditions for all subjects for whom metabolic data wasavailable in these conditions (P=0.04, not quite significant at the 5%level with the {hacek over (S)}idák correction). This is misleading,however, as the respiratory exchange ratio is notably higher for trialsin the inactive and active condition than for trials in the controlcondition. Though always below 1.1, this shift in respiratory exchangeratio implies that some anaerobic contribution is present when the braceis worn, making comparisons between the control and inactive casetenuous. It is worth noting that if S5's anomalously low demand in theinactive condition is omitted as an outlier, the difference betweenthese conditions becomes significant, as is expected from subjectivereactions to running with the additional mass.

There is no evidence against the null hypotheses that leg stiffness andknee stiffness are each unchanged by the presence of an externalparallel spring at the knee.

Subject Variation in Response to Intervention

Closer examination of Table 6 suggests that the population may bedivided into two groups according to level of training. As shown in FIG.25, trained competitive marathoners S1 and S3 appear to exhibitincreased total knee stiffness in the active condition, whilerecreational runners S2 and S4 exhibit unchanged knee stiffness despitethe external stiffness. While statistics for such a small sample must beapproached cautiously, a two sided paired t-test suggests increasedtotal knee stiffness in both absorption and generation in marathoners(P=0.07 in both cases) with no corresponding effect in recreationalrunners (P=0.80 in both cases). Ankle stiffness in generation is alsofound to increase in the active case in marathoners (P<0.01) but not inrecreational runners (P=0.50). Marathoners S1 and S3 also exhibit small(2%) reductions in metabolic demand above resting while S2 and S4 donot, though this effect is not statistically significant. Verifyingthese apparent differences in stiffness and metabolic demand based onrunner training would require subsequent investigation with largersamples of recreational and trained runners, however.

EQUIVALENTS

While this invention has been particularly shown and described withreference to various embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the scope of the invention encompassed bythe appended claims.

The relevant teachings of all references cited are incorporated byreference herein in their entirety.

What is claimed is:
 1. A method for augmenting running in a mammal,comprising the steps of: a) adaptively modulating anticipation of amaximum extension of an exoskeletal clutch linked to at least oneelastic element, the exoskeletal clutch and the elastic element being anexoskeleton attached in parallel to at least one muscle-tendon unit of aleg of the mammal and spanning at least one joint of the mammal, tothereby estimate a predicted maximum extension of the exoskeletal clutchprior to leg strike of the mammal while running; b) actuating theexoskeletal clutch in advance of the predicted maximum extension of theexoskeletal clutch, to thereby cause the exoskeletal clutch to lockessentially simultaneously with the ground strike of the leg by themammal, whereby the elastic element is engaged during a stance phase ofthe gait of the mammal while running; and c) disengaging the elasticelement prior to or during a swing phase of the gait of the mammal,thereby augmenting running in the mammal.
 2. The method of claim 1,wherein adaptively modulating anticipation of the maximum extension ofthe exoskeletal clutch includes correlating both a) a position of theexoskeletal clutch; and b) an angular velocity of the exoskeleton in asagittal plane of the mammal, with a phase of the gait cycle of themammal while running, to thereby estimate the predicted maximumextension of the exoskeletal clutch prior to leg strike of the mammalwhile running.
 3. The method of claim 2, wherein adaptively modulatinganticipation of the maximum extension of the exoskeletal clutch furtherincludes, upon or after estimating the predicted the maximum extension,correlating past positions of the exoskeletal clutch during terminalswing phase with each other, to thereby predict maximum extension of theexoskeletal clutch while running.
 4. The method of claim 3, wherein theelastic element is disengaged by: a) correlating the position of theexoskeletal clutch and the angular velocity of the exoskeleton with amid-stance phase of the gait cycle; and b) actuating disengagement ofthe exoskeletal clutch during the mid-stance phase.
 5. The method ofclaim 3, wherein correlating the past positions of the exoskeletalclutch to predict maximum extension of the exoskeleton includes using alatency compensation algorithm.
 6. The method of claim 5, wherein thelatency compensation algorithm includes a quadratic least squaresanalysis.
 7. The method of claim 5, wherein the latency compensationalgorithm includes fitting differentials of encoder readings to a lineand seeking a zero crossing.
 8. The method of claim 1, wherein theclutch is a rotary clutch.
 9. The method of claim 8, wherein the elasticelement includes a leaf spring.
 10. The method of claim 9, wherein therotary clutch links two leaf springs in series.
 11. The method of claim1, wherein the exoskeleton spans a knee joint of the mammal.
 12. Themethod of claim 11, wherein the exoskeleton further spans an ankle jointof the mammal.
 13. The method of claim 12, wherein the exoskeletonfurther spans a hip joint of the mammal.
 14. The method of claim 11,wherein the exoskeleton further spans at least one of an ankle joint anda hip joint of the mammal.
 15. The method of claim 1, wherein the clutchis a linear clutch.
 16. The method of claim 15, wherein the elasticelement is at least one member selected from the group consisting of aleaf spring, compression spring and tension spring.
 17. A method foraugmenting running in a mammal, comprising the steps of: a) adaptivelyanticipating a maximum extension of an exoskeletal clutch, to therebyestimate a predicted maximum extension of the exoskeletal clutch priorto leg strike of the mammal while running; b) actuating the exoskeletalclutch in advance of the predicted maximum extension of the exoskeletalclutch; and c) disengaging the exoskeletal clutch prior to or during aswing phase of the gait of the mammal, thereby augmenting running in themammal.